In order safely to work well above the floor, for instance doing regular maintenance such as cleaning glass or changing light bulbs, it is necessary to have a platform big enough for the worker or workers that it will carry, and mechanism to raise it up to the working level and hold it steadily there once raised. Such a piece of equipment must be very stable, especially as it is normally used a great deal and under difficult conditions.
The platforms of these devices are normally liftable to a height that is a multiple of its starting height, a standard unit having a platform height that is 6'10" at minimum and 28'2" at maximum. The minimum height is mainly dictated by the doorway size the equipment must fit through.
Thus the lifting mechanism must be able to fold up to a fairly small size. In standard lever arrangements, in particular of the lazy-tongs type, the folded up lifting mechanism, whether operated by a motor or by a winch, is very bulky. In addition such mechanisms are quite expensive to produce, as they must support heavy loads, withstand substantial torques, and at the same time fit snugly together when down.
Hence recourse is had to a telescoping mast to support such a platform. These arrangements can fold up in a very compact manner, but nonetheless are expensive to build and service. They can also be very heavy, in particular in hydraulic models.
Two-mast hydraulic arrangements, which can support relatively large platforms, are particularly difficult to use. For transport the two masts are pivoted horizontal over the platform and chassis, requiring that this chassis be asymmetrical as the one pivot must be above the other so the one mast can lie atop the other. To raise the platform it is necessary to pivot up and lock both these masts, secure the platform in place, deploy the leveling jacks and/or outriggers, and only then can the platform be raised.